1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine, and particularly, to an exhaust gas purifying apparatus having a diesel particulate filter (hereinafter referred to as “DPF”) which traps particulates, i.e., particulate matter, in exhaust gases.
2. Description of the Related Art
In an internal combustion engine, particularly, in a diesel engine, a DPF is conventionally and widely used to trap particulates or particulate matter existing in exhaust gases. Since there is a limit to the amount of particulates that can be trapped in the DPF, the amount of particulates accumulated in the DPF must be estimated. Accordingly, the regeneration process for burning the particulates is performed when the estimated amount of particulate accumulation reaches a predetermined threshold value.
Japanese Patent Laid-open No. 2003-155920 (JP '920) discloses a device which detects a pressure difference between an upstream side pressure and a downstream side pressure of the DPF, and then determines if there is a failure, such as a blockage or a leak of the DPF, based on the detected pressure difference.
The pressure difference sensor for detecting the above-described pressure difference is disposed in a passage (hereinafter referred to as “pressure difference detection passage”) which is provided for detecting the pressure difference and which connects the upstream side and the downstream side of the DPF. Accordingly, if water in the pressure difference detection passage freezes, an abnormal rise or an abnormal fall of the pressure difference may be detected. Further, when the pressure difference sensor itself freezes, the detected pressure difference may take an abnormal value. In this specification, the frozen state, including the frozen water in the pressure difference detection passage, is hereinafter referred to as “frozen state of the pressure difference sensor”. Since such a frozen state is not considered in the device disclosed by JP '920, the device may erroneously determine that the DPF has failed, i.e., is abnormal, in the frozen state of the pressure difference sensor, even though the DPF has not failed.